Usually the total stock market fund will overlap a 500 index fund, so you may be over-weighted with large cap stocks by holding both. You also may not have enough international exposure, depending on whether that matters to you. They would both be almost entirely US equities.
I checked out FSKTX at Morningstar, Over 10 years, the fund has been within 0.19 of the S&P 500. If you look over the last 1, 3, 5 years you'll notice the same thing-the returns are nearly identical. It's in the top 20% of funds in its category (which is great).
The reason it tracks so closely is that the "total market" is a bit misleading. It's not actually the total stock market including small companies. Looking at Fidelity's page you'll find this fact about the fund -
Invest in companies with market values greater than $10 billion. These funds invest in a combination of growth and value-oriented stocks.
So you're missing out on companies with a market capitalization under $10 billion, which would be most mid and small sized companies, in addition to missing out on international.
Do you have other inexpensive fund options available to you that might cover you for the kinds of companies you're missing? Or do you feel most comfortable with larger US companies?